The present invention relates to a photoelectric rangefinder for determining the distance of a target object relative to a reference plane.
There is known a photoelectric rangefinder system comprising a pair of photoelectric elements, each capable of generating an electric signal indicative of the intensity of light sensed thereby, a spatial filter supported for movement so as to mechanically scan the image forming plane of an objective lens system, and means for generating an electric signal representative of the difference in phase between the output signals from the respective photoelectric elements which results from the fact that the incident light falling on one of the photoelectric elements and that falling on the other of the photoelectric elements are transmitted thereto from different portions of the target object. This phase difference is indicative of the distance of the target object relative to the image forming plane. This type of photoelectric rangefinder system is disclosed, for example, in German Laid-open Patent Publication No. 2,330,940, Japanese Laid-open Patent Publication No. 48-60645, German Patent Publication No. 2,619,795 and Japanese Laid-open Patent Publication No. 48-81530.
It is a general belief that a rangefinder system designed to be incorporated in a compact photographic camera can operate precisely with a minimum possible consumption of electric power and can be manufactured at a relatively low cost. Despite of this general belief, the conventional rangefinder system of the construction described above includes various disadvantages and inconveniences. By way of example, in order to improve the distance measuring performance of the rangefinder system the spatial filter must scan at a relatively high speed within a tolerance of tens of microns in a direction parallel to the optical axis and also in a direction perpendicular to the longitudinal axis of the parallel slits forming the spatial filter. Employment of a mechanical drive mechanism for achieving this requirement requires a lot of energy and, if an electromagnetic drive unit is employed in place of a mechanical drive mechanism for driving the spatial filter for scanning, the power source usually employed in the photographic camera cannot provide a sufficient amount of electric power necessary to operate the electromagnetic drive unit. Moreover, the rangefinder system tends to become bulky to such an extent as to render the photographic camera bulky and expensive.
Furthermore, in the conventional rangefinder sytem, unless the scanning speed is fixed at a predetermined value, the phase difference in the electric output signals will vary even though the images of respective target objects are equally focused on the same image forming plane, thereby providing an erroneous electric signal.
In the mechanical scanning such as performed by the conventional rangefinder, the slits forming the spatial filter move reciprocately and, therefore, it is difficult to cause the slits to move at a predetermined speed. Yet, in the conventional rangefinder system wherein the spatial filter undergoes a mechanical scanning operation, a precisely positioned mask is required to fix the range of distance measurement. In addition, where the target object has a pattern of finely divided bright and dark areas, the output signals will have a complicated waveform to such an extent that a complicated signal processing circuit is required and, in the worst case, erroneous operation is likely to occur. Moreover, since the conventional rangefinder system employs mechanically movable parts operated at a relatively high speed, the rangefinder system tends to fail to operate properly.
In view of the above, the conventional rangefinder system is bulky and expensive, requiring complicated signal processing circuitry and, therefore, can hardly be practically incorporated into a compact camera.
There is also known a photoelectric rangefinder system comprising fixed and movable lens systems arranged in spaced relation to each other in a plane perpendicular to the optical axis, a slit member positioned on an image forming plane of one of the fixed and movable lens systems for passing of a portion of the rays of light passing through the corresponding fixed and movable lens systems, and a photoelectric detector element positioned behind the slit member. In this known rangefinder system, the slit member is oscillated in a predetermined cycle so that the photoelectric detector element can generate substantially alternating output signals resulting from a partial difference in brightness of a target object. These output signals are subsequently converted into electric signals having a substantially rectangular waveform.
A servo motor for controlling the direction of movement of the movable lens system and the direction of movement of the same movable lens system is controlled by a difference signal indicative of the difference in phase between the signals of the rectangular waveform and an electric signal indicative of the frequency of oscillation of the slit member or the photoelectric detector element. This type of rangefinder system is disclosed, for example, in any one of the Japanese Patent Publications No. 47-6983 and No. 47-27407, both published in 1972.
With this known rangefinder system, in the case where the brightness distribution of the target object is dense, the cycle of the rectangular wave produced by converting the output signal from the photoelectric detector element tends to be shortened to such an extent that the rangefinder system may function erroneously. Moreover, since the slit member or the photoelectric detector element is mechanically oscillated, even this rangefinder system involves such disadvantages as hereinbefore described in connection with the first mentioned known type of rangefinder system.
Furthermore, where the target object is illuminated by a source of illumination having an A.C. or pulsating component, such as noticeable under an artificial lighting condition, the output signals generated from the photoelectric detector element have a noise component superimposed thereon as shown in FIG. 3. Because of this, in the known rangefinder systems not only is the signal processing complicated, but also a complicated and expensive circuit is required.